In a conventional aircraft, the fuselage is generally constructed as a diaphragm structure comprising a skeleton of circumferential frames and longitudinal reinforcing members or stringers overlaid by a fuselage skin panel. The fuselage skin panel generally comprises a number of individual skin panels which are secured to the frame or stringers of the aircraft. The fuselage skin panel, among other things, provides an aerodynamic surface for the aircraft.
The conventional aircraft and particularly, the conventional aircraft designed for transporting passengers, generally includes a plurality of interior panels. These interior panels generally define the interior compartment or cabin of the aircraft and may include a trim or sidewall panel, a ceiling panel and a floor panel. The panels are typically supported by the frame of the aircraft. However, effort is made to decouple these interior panels from the frame assembly by spacing them apart from the fuselage frame assembly and by limiting the number of supports. This reduces the transfer of vibration from the aircraft frame assembly into the interior panels.
As the aircraft travels through the atmosphere, air turbulence is generated around the fuselage skin panels. This air turbulence acts on the skin panels setting them into several modes of vibration, including a significant lateral shear wave. These modes of vibration generally include a number of particular frequencies that take on a modal response of nodes and anti-nodes across the surface of the skin panels.
A vibrating skin panel can often act as an efficient loud speaker, radiating noise into the interior panels and into the cabin of the aircraft. Energy from the vibrating skin panels can also be transferred into the aircraft frame assembly supporting the skin panels and subsequently to the interior panels of the aircraft, also setting them into vibration. The vibrating interior panels can also act as loud speakers, generating sound which is directly transmitted into the cabin of the aircraft.
In addition to the modal frequencies incident in the fuselage due to air turbulence, there are a number of other frequencies in which the skin panels and the frame assembly are forced to vibrate. These frequencies include engine tone frequencies and air resonance frequencies. The engine tone frequencies are particular frequencies of vibration generated by the aircraft engines. These vibrations are transferred through the fuselage structure to the fuselage panels and into the interior panels of the aircraft.
Air resonance frequencies are particular frequencies of air resonance (or standing waves) formed when air resonates in the cavity between the fuselage skin panel and the spaced apart interior panels. Air resonance may also be formed between adjacent frame members within the cavity between the skin panels and the interior panels.
The circumferential frame members are generally spaced apart along the longitudinal axis of the aircraft. The ribs or members of this frame structure provides a periodic cavity between each frame member suitable for formation of standing waves at certain frequencies. The standing waves formed between adjacent frame members causes lateral vibration of the frame members parallel to the longitudinal axis of the aircraft. In addition, the frame members are coupled together through the stringers and the skin panels. Therefore, resonant vibration set up between two frame members due to airborne noise can easily be transferred to adjacent frame members and endure longer, as in tuning forks. This lateral vibration of the circumferential frame members enhances the out-of-plane vibration (perpendicular to the longitudinal axis of the aircraft) of the fuselage skin panels and increases the sound radiated into the interior panel and cabin.
A common prior art solution for attenuating the transmission of noise into the cabin of an aircraft utilizes insulation blankets. These insulation blankets generally incorporate both thermal and acoustic insulation and are placed into the air space or cavity between the fuselage skin panels and the interior panels. Although somewhat effective in attenuating higher frequency noise, insulation blankets are relatively ineffective at attenuating low frequency noise. This lower frequency noise tends to transmit the majority of noise into the aircraft cabin.
In addition to not significantly attenuating low frequency noise, prior art insulation blankets have other shortcomings. For example, these fuselage insulation blankets often become wet and retain moisture during normal flight operation as a result of condensation on the aircraft fuselage skin panels. In addition to increasing the overall weight of the aircraft, these wet insulation blankets increase the corrosion of the aircraft. Additionally, airlines have experienced passenger and service door malfunctions due to frozen insulation blankets in the door.
Another common prior art solution for attenuating the transmission of noise into the cabin of an aircraft utilizes passive damping devices such as a viscoelastic panels or skin panels having an increased density towards the frame members. These panels are attached to the fuselage and act to dampen the incident vibration. However, these devices are only effective at dampening at particular frequencies. In addition, these devices do little to attenuate sound radiated into the interior cabin of the aircraft due to the air resonance previously discussed. These passive devices can also add considerable weight to the aircraft.
A need has existed in the prior art for a conventional aircraft or other reinforced skin structure which provides improved noise attenuation within the fuselage. There is also a need for such an aircraft or other reinforced skin structure which effectively attenuates noise transmitted into the interior of the craft due to air resonance in the cavity between the fuselage or outer skin panels as well as noise transmitted into the aircraft due to air resonance and vibration in the cavities between the outer skin panels and adjacent frame members. There is also a need for such an aircraft or other reinforced skin structure which has improved thermal and acoustic insulation which prevents the capture and retention of condensation. There is also a need for a method and apparatus of attenuating noise transmitting into the interior of an existing conventional aircraft which is lightweight, non-toxic, inexpensive and simple to install.